Tuned microwave wattmeter



April 29, l1947.

H. EQ WEBBER TUNED MICROWAVE WATTMETER 2 Sheets-Sheet 1 Filed Dec. 15, 1945 INVENTOR HUGH E. WEBBER ATTORNEY April 29, `1947. H. E. WEBBER 2,419,613

TUNED MICROWAVE WATTMETER Filed Dec. 13, 1943 2 Sheets-Sheet 2 VNO FIG. 4.

FIG. 5.

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INVENTOR ATTORNEY Patente-d Apr. 29, 1947 TUNED MICROWAVE WATTMETR Hugh E. Webber, Williston Park, N. Y., assignor to Sperry Gyroscope Company, Inc., a corporation of New York Application December` 13, 1943,l Serial No. 514,111

Claims.

The present application is a continuation-inpart of my copending yapplication Serial No. 513,164, filed December, 1943.

The present invention relates to tuned micro- 'wave' Wattmeters.

An object of my invention is to provide methods and apparatus for measuring power at frequencies inV the microwave region.

More particularly, itis an object of my invention to provide a hollow pipe-waveguide type of wattmeter.

Another object of my invention is to provide an improved arrangement for dis'sipating energy in' a Wave guide, for avoiding reflections from energy absorbing devices in the wave guide and for matching impedances in a wave guide.

It is an object to provideimproved arrangements for coupling a power absorbing wire or lament to a wave guide.

Another object is to provide a microwave wattmeter having ambient temperature compensation.

Still another object of the invention is to provide wave guide apparatus which may be tuned by an adjustable probe. I y

A further object of the invention is to provide a mounting arrangement for a tuning probe with both axial and transverse adjustment,

Other and further objects and advantages of the invention will become apparent as the description proceeds.

In carrying out the invention in its preferred form, microwave energy, the power of whichY is to be measured, is conveyed through a hollow pipe wave guide type of transmission line and a powerabsorbing hot Wire such as a barretter wire ismounted athwart the wave guide.

Means are provided for matchingthe impedance of the barretter wire to the line.

For ascertaining the radio frequency power consumed in the barretter wire, direct-current is supplied to it, means are provided for measuring the direct current power input,v and means are provided for measuring the resistance of the wire and maint-aining it vconstant by adjusting the supply of direct current. Variations in the'direct current power required to maintainconstant resistance then represent variations in radio frequency power supplied to the wire bythe transmission line. l

A better*understandingV of the invention will be afforded lby' the 'following' detailed'l description,A

considered in conjunction with the 'accompanying drawings, wherein:

Fig. 1 is a vview kof a @barretter wireholder for av -microwave wattmeter shown as cut by a longitudinal plane;

Fig. 2 is a fragmentary view in longitudinal section of a portion of the apparatus of Fig. l represented as cut by a plane 2-2 of Fig. l;

Fig. 3 is a circuit diagram of a Ibridge circuit which may be employed in conjunction with the apparatus of Fig. 1 `for maintaining the directcurrent (or low-frequency alternating-current) resistance of the variable Wire and measuring the direct current power input thereto;

Fig.Y 4 is a longitudinal view of a barretter wire holder employing an adjustable probe for tuning;

Fig. 5 is a cross-sectional view of a portion of the apparatus of Fig. 4 cut by the plane 5 5 indicated in Fig. 4; and

Fig. 6 is a fragmentary longitudinal-sectional view of a portion oftheapparatus `of Fig. 4 as cut by a plane 6-'6 in Fig.` 4.

Like'freference characters are utilized throughout the drawings toidesignate like parts-. Y

. In my aforesaid parent application, I have described a high frequency wattmeter employing a bridge "circuit for measuring the direct current resistance of a barretter wire to which bothalternating and directcurrents are supplied. A similar bridge circuit may. be employed in connection with the measurement of power of ultra high frequency energy conveyed through a hollow pipe wave guide type of transmission line. In my aforesaid application I have described suitalble holders for loarretter wires where the alternating current energy was conveyed through coaxial transmission'lines. I also described suitable constructions for'suchloarretterwires, particularly in connection with the measurement of low power, in which case a iine' barretter wire' was required.

In the present application wire holders are described for circuits in which radio frequency energy isr supplied through waveguide transmission lines.

summarizingV from my aforesaid application, the bridge, as illustrated in Fig. 3 of the present application, comprises four arms II, I2, I3 and I4,

. with a diagonal arm I5 and a current source I6.

represented Aschematically in Fig. 3 'and shown inA stant resistance, and the resistor I1 is so chosenV as to have a resistance which, when added to the resistance of the milliameter I8, will produce a total resistance l.for the bridge arm I3 of a proper value to balance the bridge when the barretter wire I9 has a predetermined resistance corresponding to the maximum power input and temperature considered desirable for the barretter wire I9.

If ambient temperature compensation is desired, one of the resistors such as the bridge arm I2 will take the form oi a compensating resistor mounted in proximity to the barretter wire' unit la within the wire holder 2|.

` The wire holder shown in Fig. 1 comprises a self-shielding type of transmission line such as a rectangular hollow pipe wave guide 3| open at the left-hand end and having a choke coupling 32 mounted thereon for connecting the open end of the line to an input line and corresponding choke coupling (not shown). The narrow dimension of the wave guide Slis that shown in Fig. 1, and the wide'dimensionis that shown in Fig. 2. Suitablesupporting means are provided for mounting the barretter wire unit I4 athwart the wave guide 3| parallel to the narrow dimension thereof.

For enabling a direct current connection to be made Ato the barretter Wire unit lid-without interposing radio frequency impedance in the line 3| or causingreflections therein, a short transverse line or lateral tuning stub 33 isA provided. In

the arrangement shown, the tuning stub 33 is of the concentric, substantially circular transmissionrline type. `For tuning the lines and matching' the impedance of the line to that of the barretter wire I9, the wave guide 3| is provided with a line-length adjusting mechanism 34 closing the right-hand end of the line 3l, and the stub line 33 is provided with'a corresponding ad justing mechanism 35.

The choke coupling 32 may be one of the type already known to those skilled in the art and illustrated and described in greater detail in the copending application of William A. Hayes, Serial No. 503,613, led September 24,Y 1943.` Briefly, f

such a choke couplingv comprises a circular block having a rectangular opening fitted overV the line 3| and having wave trap slots 36 cut therein, the slots 36 beingV a quarter-wave deep and a quarter wave vdistant from the edge of the line 3|.

- The adjusting mechanism 34 closing the righthand end 'of the wave guide 3| comprises a4 quarter-wave trap-31 mounted within the guide 3l to form a short-circuiting plug which is adjustable in longitudinal position.` The trap 31 comprises a-hollow rectangular-shaped member with a base' 38`slightly smaller than the internal dimensions of the wave guide 3| and having 'longitudinally extending sides or web portions 39- one quarter wave in length which serve as a wave trap and assure substantially z'ero impedance closure of the waveguide transmission line 3| substantially at the surface 38. For supporting the trap member 3l, a positioning means is provided including a sliding block 4i having a slip fit within the wave guide 3|, a supporting rod 52 staked into the base 33 of the trap member 3l and also inserted with a drive fit into an opening in the sliding block Gl, and adjusting means such as a micrometer screw arrangement. The micrometer screw arrangement includes a block 43 secured in the "endoi ythe wave guide 3|l and having a threaded opening, a hollow screw lili having a `knurled head d5, an internal sore7 llt having a "slotted head fil and having a tapped opening E Yso -at the opposite end thereof, and a threaded extension di. of the rod 42 tting the tapped opening fia of the internal screw The screw head d5 is counterlooredV to receive the screw head 5l with a loose fit. Set screws 5) are provided to prevent the internal screw l5 from turning with respect to the s rew 59 and the block 6I.

The stub line 33 includes an outer hollow cylinder 5I and an inner conductor rod 52, The line-length adjusting' mechanism 35 closing the end oiv 'the line'33 'includes an axially movable hollow cylindrical block 53 fitting the center rod 52 and oi siightly smaller diameter than the internal surface of the cylinder 5i, having a t0- roidal or hollow cylindrical downward projection carrying two wavetrap members 55 secured and electrically connected-to the rmember 54 at the lower `end and extending upwardgwith clearance so as to 4avoid contact with the internal rod 52, the outer cylinder 5i, the supporting projection 54, or a shoulder 56 of the slidable member 53. `For slidably 'supporting the member 53 within the hollow cylinder 5I, ring-shaped shoe members lcomposed'ofsuitable material such as brass are provided; For enabling the rod 52 to be supportedat the upper end, it is formed with a head 53, "a capl 59 is provided which is adapted to clamp the head 58 against the top of the cylinder 5i. For supporting the intermediate or lower portion of the rod` 52 according vto the position of the member 5e, a relatively close slide iit is made between the memberi and the rod 52.

For xing the axial position ofthe member 53 with respect-to the cylinder 5|, thefouter surface of the cylinder 5i is threaded, Apair of pins 6|! is inserted in the? member 53 with a drive-fit, and a slotted positioning nut d!y is provided. .The positioning nut 6I comprises a cup-shaped member 52 with an annular block 63 fastened therein, for example, by a machine screw 64 and spaced sufficiently from the base of thecup-shaped memberto form a slot @5 for receiving the pins 53. The innerl surface of either the cup-shaped member 552 or thev annular block 53 may be threaded to engage the outer surface of the hollow cylinder 5I. If desired, a ring shaped lock nut @E may also beprovided.' The cylinder 5| is iitted into the top surface of the wave guide 3| and mechanically and electrically connected theretoV as by' `soldering or weldingja smooth internal joint being employed so as to maintain a flat internal surface for the wave guide 3|;

The lowerend of the rodv 32 is countersunk and may also be slotted at one side to 'form a retainingv spring socketl'! for the upper end of the barretter wire unit I4. v v

- The unit I4, as shown, comprises an insulating tubular body 68 carrying apair of ferrules 69 closing the ends of the tube 68 and electrically connected tothe ends-ofthe barretter wire I9. For' supporting the lower end of the barretter 'unit I4, a second spring clip or socket 1I is provided, and for making direct currentvconnections to the wire I9, a directcurrentconnector I2 is provided.v The supporting means for the lower socket 1I yandthe connector 12 include a hollow block 113 of substantially circular cross-section, a cupshaped fitting 14, an insulating shell 15, a lock nut 1li, and insulating bushings 11 and 19.

Forthe purpose of avoiding the interposition of a .discontinuity in the microwave transmission line 3|, a wave-trap in the form of a shouldered hollow cylinder 19 is provided. The block 13 is Iitted .into the lower surface of the wave guide 3| and mechanically and electrically connected thereto, and the member 19 is tted into the top end of the member 13 so as to leave a flat lower surface of the wave guide 3|. The member 19 is spaced from the lower barretter socket 1I and has an axial length of approximately a quarter lwave so as to forma quarter-wave trap.

The hollow cylindrical quarter-wave trap 19 includes a flange 99 and a hollow cylindrical downwardly extending portion 8|, the length of which is substantially equal to a quarter wave,

and the portion 8| is spaced from the terminal socket 1I.

The member 14 has a groove 'E4' formed in the outer surface thereof to form a spool for receiving Ypin 83 is electrically connected to the threaded ring 82 by a conductor 86. The center pin 34 is connected to the barretter wire spring socket 1| by a conductor 81, and the pin 95 is connected to one end of the compensating resistor wire |52 by a conductor 98, the other end of the wire I2 being connected to the metallic spool 13. It will be understood that insulated wire, such as enamel-covered wire, is utilized to form the resistor I2, and the surface of the groove 14 may be insulated by a suitable coating such as a coating of enamel, porcelain, or aluminum oxide to provide further safeguards against short-circuiting turns of the resistor I3.

It will be understood that a suitable connector (not shown) cooperating with the connector 12 is utilized for connecting the pins 93, 94 and 85 to corresponding conductors or terminals of the bridge shown in Fig. 3. Thus, the bridge terminal 83 is represented in Fig. 1 by the connection of the conductor 89 to the metallic housing B?! of the connector 12 and the other metallic parts of the wire holder are represented in Fig. 3 by a conductor 39. The pin 84 is connected to the bridge terminal 25 and the pin 95 is connected to the terminal 24 of the bridge shown in Fig. 3.

A direct current connection including Athe wire |9 is thus formed from .the pin 84 through the conductor 81, the socket terminal 1I, the ferrule 69, the wire I 9, the second ferrule 69, the spring socket contact |31, the post 52, the head 59, the cap 59, the outer cylinder I of the stub line 33, the wave guide 3|, the members 13 and 14, the connector ring 82, the connection 21, and the conductor 26 back to the pin 83. Likewise, a connection to the compensating resistance arm I2 is formed from the pin S5 through the conductor 88, the wire I3, the grounded end of the wire, the member 14, the connector ring 82, the conductor 96 and the pin l93. The compensating wire I2 is thermally connected to the metal surrounding the barretter wire I9, through the members 14, 15, 19, 3| and 33.

It will be observed that the lower spring socket member 14 threaded into the member 13. The ybarretter wire unit I4 may be installed or replaced by opening the holder at either the upper .or the lower end. 'For example, if the holder isto be opened at the lower end, the lock-nut 16 and the member' 14 are unscrewed; or if the holder is to be .opened at the upper end, the cap 59 is vu'nscrewed and the rod 52r is withdrawn to give access to the'unit I4. Preferably, the spring pressure of the spring contact 61 is'rnade greater'than .that of the'spring'contact 1I so that the unit I4 will be withdrawn with the rod 52.

When the position of the wave trap member 31 is adjusted by rotation of the knurled screw head 45, the internal screw remains fixed in angular position. The head 41 of the internal screw 46 limits leftward motion of the block 4I and the end of the hollow screw 44 limits motion of the block 4I to the right. V

In the arrangement of Fig. 4 the supporting structure for the barreter unit I4 is made more compact by omitting the stub line 33 and instead providing a movable-probe adjusting mechanism 9| for tuning theline and matching impedance. As shown in Fig. 4, a rectangular hollow pipe Wave guide 3| is provided, having line-length adjusting mechanism 34', upper and lower barretter unit supporting members 92 and 93, and the probe adjusting mechanism 9|.

The mechanism 34 includes the wave trap member 31 carried by a sliding block 4 I ,the screw 94 rigidly secured to the block 4I, an adjusting ring nut 95 and a lock nut 96. The member 31 is'rigidly secured to the block 4I through a supporting post 91, and a ring nut retaining frame 98 is provided. The latter comprises a yokeshaped block 99 (Fig. 6) secured inthe end of the/wave guide 3| in a suitable manner as by means of screws IIII and a slot closing strap |02 screwed tothe ends of the block 99 to form a nut retaining slot |03.

The barretter wire supporting members 92 and 93. are composed of a conducting material such as brass and are fitted into upper and lower openings in the wave guide 3|', being rigidly secured thereto by soldering, brazing, or the like. The members 92 and 93 have openings for receiving the unit I4, and flanged wave'trap members |94 and |95 are tted into the openings so as to leave flat upper and lower internal surfaces in the wave guide 3|'. The members |94 and |95 are each a quarter wave in length, substantially, and are provided with sufficient clearance from the unit I4 and from the internal surfaces of the members 92 and 93 to form wave traps, thereby avoiding any discontinuities in the wave guide 3| introduced by the presence of the opening for the barretter wire unit I4.

For securing the unit I4 in the members 92 and 93, an upper spring clip or socket |06 is provided which is tted in the end of the member 92. A lower insulating socket member |91 is provided which is fitted into the member 93. A threaded cap |98 is provided for securing the clip |99 in the member 92, and a positioning screw |99 may be adjustably mounted in the member |96. A member III forming a lower cap and connector housing is threadedV onto the member 93. To form a connector, an insulating bushingv I I2 ris mounted in the housing III and a center pin I I3 is centrally secured in the bushing II2. An. insulating spacing cylinder II4 is provided for enabling the insulator IIJ'I to be secured'in the member 93 by means lof the insulator |I2 and the connector housing II.

The barretter unit I4 is provided with a downwardly extending terminal -pin ||5, and a spring contactor IIB adapted to engage the pin ||5 is secured to the top end of the connector pin ||3.

As shown, the barretter wire unit I4 is of the Wollaston wire type having a silver-coated wire II'I electrically connected between the lower terminal pin ||5 and an upper terminal pin I|8, the center portion I9 of the coated wire being etched away to leave a ine lament of platinum or other resistant material which forms the actual resistance of the unit and corresponds to the resistor I9 of Fig. 3.

The probe adjusting mechanism 9| comprises a pair of channeled members I 2| and |22 adapted to fit slidably over the upper and lower external surfaces of the wave guide 3|', a hollow post |23 tted'into the channel block I 2 I, a hollow nut |24 threaded to engage the outer surface ofthe post |23, and a center post |25.

A top opening |26 is provided in the upper wall of the wave guide 3| of sufcient length to allow longitudinal adjustment of the probe mechanism. Clamping screws |2'I are provided for adjustably securing the probe mechanism 9| in the desired position along the wave guide 3 I (Fig. 5). The center post |25 has a head |28 secured to the upper end of the hollow nut |24 and has a lower end |29 of reduced diameter to form a probe adapted to extend into the hollow wave guide 3 I The nut |24 is provided with a knurled head |3| at the upper end. The hollow post |23 has a reduced diameter opening fitting the probe |29, and a large diameter opening providing ample clearance for the larger diameter portion of the post |25. Rotation of the hollow nut |24 carrying the probe |29 serves for adjustment of the axial position of the probe |29, and the dimensions are such that the probe |29 may be moved a considerable distance into the wave guide 3| or substantially withdrawn therefrom.

The variation in the position of the probe |29 serves to match the impedance of the line ISI to the resistance wire ||9 by varying in magnitude and phase the reections from the probe |29 in such a manner as to cancel any reections from wire H9.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a microwave wattmeter, in combination, a rectangular hollow wave guide adapted to receive microwave energy at one end and having an adjustable position closure at the other end, and a barretter mounting assembly intermediate the ends of the line, said mounting assembly including a concentric adjustable tuning stub line laterally extending in one direction from the wave guide and a connector assembly laterally extending in the opposite direction from said wave guide with a barretter unit mounted between said stub line and the .connector assembly, said stub line including a center conductor with a connection socket for the barretter unit in the end thereof and said connector assembly including a second connection socket for said barretter unit insulated from the wave guide.

2. Apparatus as in claim 1, in which a hollow cylindrical quarter-wave trap is provided surrounding the second connection socket for the barretter unit.

3. Apparatus as in claim 1, including a com-y pensating resistor mounted in thermal connection with the wave guide for providing an ambient temperature compensation.

4. In a microwave wattmeter a hollow wave guide adapted to receive microwave energy, said guide having an opening at the center thereof with a barretter Wire unit mounted therein athwart the wave guide, and members on either side of the wave guide for supporting said barretter wire unit including wave trap means for preventing electrical discontinuity in the wave guide.

5. In combination, a hollow-pipe wave guide, a barretter wire mounted in the wave guide, and a temperature compensating resistor in thermal association with the wave guide, said barretter wire and said resistor being so connected in a bridge circuit that said resistor provides ambient temperature compensation for said barretter wire.

6. In a microwave wattmeter, a hollow pipe wave guide having an input end through which it is adapted to receive microwave energy to be measured, a barretter wire mounted in the wave guide, and an impedance-matching probe mounted between the input end of the wave guide and the barretter wire.

7. Apparatus as in claim 6, in which the probe is provided with longitudinal adjustment for determining its position along the wave guide and axial adjustment for determining its degree of insertion into the wave guide.

8. In combination, a hollow pipe wave guide having opposite walls with openings therein, a barretter wire unit mounted athwart the wave guide extending through said openings, and choke means for said openings arranged to prevent discontinuities in said wave guide.

9. Apparatus as in claim 8, wherein one of said choke means is a quarter wave trap.

10. In a microwave wattmeter, in combination, a self-shielding transmission line having an input end, a barretter wire therein, and an impedance-matching probe for said line located between said input end and said barretter wire.

HUGH E. WEBBER.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,081,044 Runaldue May 18, 1937 1,407,147 Goodwin Feb. 21, 1922 1,612,076 Taylor Dec. 28, 1926 2,199,247 Rich et al Apr. 30, 1940 2,296,678 Linder Sept. 22, 1942 2,088,749 King Aug. 3, 1937 2,151,157 Scheikunolf Mar. 21, 1939 2,197,122 Bowen Apr. 16, 1940 2,365,207 Moles Dec. 19, 1944 2,106,713 Bowen Feb. 1, 1938 2,232,179. King Feb. 18, 1941 

